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  <div class="section" id="numpy-choose">
<h1>numpy.choose<a class="headerlink" href="#numpy-choose" title="Permalink to this headline">¶</a></h1>
<dl class="function">
<dt id="numpy.choose">
<code class="sig-prename descclassname">numpy.</code><code class="sig-name descname">choose</code><span class="sig-paren">(</span><em class="sig-param">a</em>, <em class="sig-param">choices</em>, <em class="sig-param">out=None</em>, <em class="sig-param">mode='raise'</em><span class="sig-paren">)</span><a class="reference external" href="https://github.com/numpy/numpy/blob/v1.18.1/numpy/core/fromnumeric.py#L311-L433"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#numpy.choose" title="Permalink to this definition">¶</a></dt>
<dd><p>Construct an array from an index array and a set of arrays to choose from.</p>
<p>First of all, if confused or uncertain, definitely look at the Examples -
in its full generality, this function is less simple than it might
seem from the following code description (below ndi =
<code class="xref py py-obj docutils literal notranslate"><span class="pre">numpy.lib.index_tricks</span></code>):</p>
<p><code class="docutils literal notranslate"><span class="pre">np.choose(a,c)</span> <span class="pre">==</span> <span class="pre">np.array([c[a[I]][I]</span> <span class="pre">for</span> <span class="pre">I</span> <span class="pre">in</span> <span class="pre">ndi.ndindex(a.shape)])</span></code>.</p>
<p>But this omits some subtleties.  Here is a fully general summary:</p>
<p>Given an “index” array (<em class="xref py py-obj">a</em>) of integers and a sequence of <em class="xref py py-obj">n</em> arrays
(<em class="xref py py-obj">choices</em>), <em class="xref py py-obj">a</em> and each choice array are first broadcast, as necessary,
to arrays of a common shape; calling these <em>Ba</em> and <em>Bchoices[i], i =
0,…,n-1</em> we have that, necessarily, <code class="docutils literal notranslate"><span class="pre">Ba.shape</span> <span class="pre">==</span> <span class="pre">Bchoices[i].shape</span></code>
for each <em class="xref py py-obj">i</em>.  Then, a new array with shape <code class="docutils literal notranslate"><span class="pre">Ba.shape</span></code> is created as
follows:</p>
<ul class="simple">
<li><p>if <code class="docutils literal notranslate"><span class="pre">mode=raise</span></code> (the default), then, first of all, each element of
<em class="xref py py-obj">a</em> (and thus <em class="xref py py-obj">Ba</em>) must be in the range <em class="xref py py-obj">[0, n-1]</em>; now, suppose that
<em class="xref py py-obj">i</em> (in that range) is the value at the <em class="xref py py-obj">(j0, j1, …, jm)</em> position
in <em class="xref py py-obj">Ba</em> - then the value at the same position in the new array is the
value in <em class="xref py py-obj">Bchoices[i]</em> at that same position;</p></li>
<li><p>if <code class="docutils literal notranslate"><span class="pre">mode=wrap</span></code>, values in <em class="xref py py-obj">a</em> (and thus <em class="xref py py-obj">Ba</em>) may be any (signed)
integer; modular arithmetic is used to map integers outside the range
<em class="xref py py-obj">[0, n-1]</em> back into that range; and then the new array is constructed
as above;</p></li>
<li><p>if <code class="docutils literal notranslate"><span class="pre">mode=clip</span></code>, values in <em class="xref py py-obj">a</em> (and thus <em class="xref py py-obj">Ba</em>) may be any (signed)
integer; negative integers are mapped to 0; values greater than <em class="xref py py-obj">n-1</em>
are mapped to <em class="xref py py-obj">n-1</em>; and then the new array is constructed as above.</p></li>
</ul>
<dl class="field-list">
<dt class="field-odd">Parameters</dt>
<dd class="field-odd"><dl>
<dt><strong>a</strong><span class="classifier">int array</span></dt><dd><p>This array must contain integers in <em class="xref py py-obj">[0, n-1]</em>, where <em class="xref py py-obj">n</em> is the number
of choices, unless <code class="docutils literal notranslate"><span class="pre">mode=wrap</span></code> or <code class="docutils literal notranslate"><span class="pre">mode=clip</span></code>, in which cases any
integers are permissible.</p>
</dd>
<dt><strong>choices</strong><span class="classifier">sequence of arrays</span></dt><dd><p>Choice arrays. <em class="xref py py-obj">a</em> and all of the choices must be broadcastable to the
same shape.  If <em class="xref py py-obj">choices</em> is itself an array (not recommended), then
its outermost dimension (i.e., the one corresponding to
<code class="docutils literal notranslate"><span class="pre">choices.shape[0]</span></code>) is taken as defining the “sequence”.</p>
</dd>
<dt><strong>out</strong><span class="classifier">array, optional</span></dt><dd><p>If provided, the result will be inserted into this array. It should
be of the appropriate shape and dtype. Note that <em class="xref py py-obj">out</em> is always
buffered if <em class="xref py py-obj">mode=’raise’</em>; use other modes for better performance.</p>
</dd>
<dt><strong>mode</strong><span class="classifier">{‘raise’ (default), ‘wrap’, ‘clip’}, optional</span></dt><dd><p>Specifies how indices outside <em class="xref py py-obj">[0, n-1]</em> will be treated:</p>
<blockquote>
<div><ul class="simple">
<li><p>‘raise’ : an exception is raised</p></li>
<li><p>‘wrap’ : value becomes value mod <em class="xref py py-obj">n</em></p></li>
<li><p>‘clip’ : values &lt; 0 are mapped to 0, values &gt; n-1 are mapped to n-1</p></li>
</ul>
</div></blockquote>
</dd>
</dl>
</dd>
<dt class="field-even">Returns</dt>
<dd class="field-even"><dl class="simple">
<dt><strong>merged_array</strong><span class="classifier">array</span></dt><dd><p>The merged result.</p>
</dd>
</dl>
</dd>
<dt class="field-odd">Raises</dt>
<dd class="field-odd"><dl class="simple">
<dt><strong>ValueError: shape mismatch</strong></dt><dd><p>If <em class="xref py py-obj">a</em> and each choice array are not all broadcastable to the same
shape.</p>
</dd>
</dl>
</dd>
</dl>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><a class="reference internal" href="numpy.ndarray.choose.html#numpy.ndarray.choose" title="numpy.ndarray.choose"><code class="xref py py-obj docutils literal notranslate"><span class="pre">ndarray.choose</span></code></a></dt><dd><p>equivalent method</p>
</dd>
<dt><a class="reference internal" href="numpy.take_along_axis.html#numpy.take_along_axis" title="numpy.take_along_axis"><code class="xref py py-obj docutils literal notranslate"><span class="pre">numpy.take_along_axis</span></code></a></dt><dd><p>Preferable if <em class="xref py py-obj">choices</em> is an array</p>
</dd>
</dl>
</div>
<p class="rubric">Notes</p>
<p>To reduce the chance of misinterpretation, even though the following
“abuse” is nominally supported, <em class="xref py py-obj">choices</em> should neither be, nor be
thought of as, a single array, i.e., the outermost sequence-like container
should be either a list or a tuple.</p>
<p class="rubric">Examples</p>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">choices</span> <span class="o">=</span> <span class="p">[[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span> <span class="p">[</span><span class="mi">10</span><span class="p">,</span> <span class="mi">11</span><span class="p">,</span> <span class="mi">12</span><span class="p">,</span> <span class="mi">13</span><span class="p">],</span>
<span class="gp">... </span>  <span class="p">[</span><span class="mi">20</span><span class="p">,</span> <span class="mi">21</span><span class="p">,</span> <span class="mi">22</span><span class="p">,</span> <span class="mi">23</span><span class="p">],</span> <span class="p">[</span><span class="mi">30</span><span class="p">,</span> <span class="mi">31</span><span class="p">,</span> <span class="mi">32</span><span class="p">,</span> <span class="mi">33</span><span class="p">]]</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">np</span><span class="o">.</span><span class="n">choose</span><span class="p">([</span><span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">],</span> <span class="n">choices</span>
<span class="gp">... </span><span class="c1"># the first element of the result will be the first element of the</span>
<span class="gp">... </span><span class="c1"># third (2+1) &quot;array&quot; in choices, namely, 20; the second element</span>
<span class="gp">... </span><span class="c1"># will be the second element of the fourth (3+1) choice array, i.e.,</span>
<span class="gp">... </span><span class="c1"># 31, etc.</span>
<span class="gp">... </span><span class="p">)</span>
<span class="go">array([20, 31, 12,  3])</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">np</span><span class="o">.</span><span class="n">choose</span><span class="p">([</span><span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">],</span> <span class="n">choices</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">&#39;clip&#39;</span><span class="p">)</span> <span class="c1"># 4 goes to 3 (4-1)</span>
<span class="go">array([20, 31, 12,  3])</span>
<span class="gp">&gt;&gt;&gt; </span><span class="c1"># because there are 4 choice arrays</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">np</span><span class="o">.</span><span class="n">choose</span><span class="p">([</span><span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">],</span> <span class="n">choices</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">&#39;wrap&#39;</span><span class="p">)</span> <span class="c1"># 4 goes to (4 mod 4)</span>
<span class="go">array([20,  1, 12,  3])</span>
<span class="gp">&gt;&gt;&gt; </span><span class="c1"># i.e., 0</span>
</pre></div>
</div>
<p>A couple examples illustrating how choose broadcasts:</p>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">a</span> <span class="o">=</span> <span class="p">[[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">],</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">],</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">]]</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">choices</span> <span class="o">=</span> <span class="p">[</span><span class="o">-</span><span class="mi">10</span><span class="p">,</span> <span class="mi">10</span><span class="p">]</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">np</span><span class="o">.</span><span class="n">choose</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">choices</span><span class="p">)</span>
<span class="go">array([[ 10, -10,  10],</span>
<span class="go">       [-10,  10, -10],</span>
<span class="go">       [ 10, -10,  10]])</span>
</pre></div>
</div>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="c1"># With thanks to Anne Archibald</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">a</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">])</span><span class="o">.</span><span class="n">reshape</span><span class="p">((</span><span class="mi">2</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">1</span><span class="p">))</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">c1</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span><span class="o">.</span><span class="n">reshape</span><span class="p">((</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">1</span><span class="p">))</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">c2</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="o">-</span><span class="mi">2</span><span class="p">,</span> <span class="o">-</span><span class="mi">3</span><span class="p">,</span> <span class="o">-</span><span class="mi">4</span><span class="p">,</span> <span class="o">-</span><span class="mi">5</span><span class="p">])</span><span class="o">.</span><span class="n">reshape</span><span class="p">((</span><span class="mi">1</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">5</span><span class="p">))</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">np</span><span class="o">.</span><span class="n">choose</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="p">(</span><span class="n">c1</span><span class="p">,</span> <span class="n">c2</span><span class="p">))</span> <span class="c1"># result is 2x3x5, res[0,:,:]=c1, res[1,:,:]=c2</span>
<span class="go">array([[[ 1,  1,  1,  1,  1],</span>
<span class="go">        [ 2,  2,  2,  2,  2],</span>
<span class="go">        [ 3,  3,  3,  3,  3]],</span>
<span class="go">       [[-1, -2, -3, -4, -5],</span>
<span class="go">        [-1, -2, -3, -4, -5],</span>
<span class="go">        [-1, -2, -3, -4, -5]]])</span>
</pre></div>
</div>
</dd></dl>

</div>


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